This should be under siebel-admin-l forum. Anyway heres the steps to
troubleshoot
1. If you are on unix check the filesystem space where your siebel is
installed
df -k. or if you are on Windows just go to windows explorer and see if the
space is critical
In both cases delete the logarchive and log files to reclaim space
2. Now this error is again due to the corruption of
$SIEBEL_ROOTDIR/svc.siebsrvr.enterprisename:si ebelservername file
3. do this to recreate this file
siebctl -r specifysiebelroot -a -g "-g gatewayname -e enterprisename -s
siebelservername"
This shoud recreate the svc file
Then try starting your siebel server again.
Please check the exact syntax for the siebctl command.

Yes I did check the disk space and have re-created the svc file and same issue.

Now, I did some further research and I have a feeling this could be memory related. We only have 1.75 Gb total RAM and 800 MB free when starting SIebel server. Is this enough RAM for Siebel server to build the shm file? We have never been able to get this file built yet.

We have Linux ES 5.1 with Siebel 8.1. Now we had issues with 8.1 during install and config so we decxided to try 8.0 and it was much cleaner during install and config...less buggy. But is it ok to install Siebel 8.1 on Linux ES 5.1? We checked recommendations and it states Linux 4.0

As long as Linux 5.1 is supported by Oracle for Siebel 8.+ it should be
fine. I dont see why you cannot.
Now you say its not building the shm file at all
How are you starting the server.
Did you try with this command
start_server -r <siebelroot> -e siebelenter -g gatewayservername:port
siebellogicalservername
Also did someone manually edit the svc file. If yes then it will fail
Please delete the svc file then recreate it using siebctl and then run the
above command for starting the siebel server
also
before you start the siebel server above ensure you source siebenv.sh or
siebsnv,csh or equivalent bash file under
siebelrootdir
Regards
Rama Iyer
CRM Support
Ext 13152